Nonetheless, the specification of \(P_{\lambda }(\psi )\)  is general, and fusion penalties can be included or omitted according to subject matter knowledge. Additional structure can even be imposed by fusing the baseline hazard parameters. For example, under the Markov transition specification a penalty of the form \(\sum _{j=1}^{k_3} p_{\lambda _3}(|\phi _{2j}-\phi _{3j}|)\)  could regularize the model towards having \(h_{02}(t_2)=h_{03}(t_2)\) . [1]} call this the `restricted' illness-death model corresponding to the baseline hazard of the terminal event being equal before and after the non-terminal event. This imposes additional structure at the cost of increasing the dimension of the regularization parameter space, so in this paper we only further consider penalization of \(\beta \)  and a two-dimensional regularization parameter \(\lambda = (\lambda _1,\lambda _2)\) .
